UNENE Chemistry Primer

1
UNENE Chemistry Primer

• Lecture 11
• Properties of Solutions
• Derek Lister and William CookUniversity of New
  Brunswick

Course Textbook Chemistry, The Central Science,
10th edition, Pearson Education Inc.,
2006 Theodore L. Brown, H. Eugene LeMay Jr. and
Bruce E. Bursten
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Solutions

• Solutions are homogeneous mixtures of two or more
  pure substances.
• In a solution, the solute is dispersed uniformly
  throughout the solvent.

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Solutions

• The intermolecular forces between solute and
  solvent particles must be strong enough to
  compete with those between solute particles and
  those between solvent particles.

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How Does a Solution Form?

• As a solution forms, the solvent pulls solute
  particles apart and surrounds, or solvates, them.

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How Does a Solution Form?

• If an ionic salt is soluble in water, it is
  because the ion-dipole interactions are strong
  enough to overcome the lattice energy of the salt
  crystal.

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Energy Changes in Solution

• Simply put, three processes affect the energetics
  of the process
• Separation of solute particles
• Separation of solvent particles
• New interactions between solute and solvent

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Energy Changes in Solution

• The enthalpy change of the overall process
  depends on ?H for each of these steps.

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Why Do Endothermic Processes Occur?

• Things do not tend to occur spontaneously (i.e.,
  without outside intervention) unless the energy
  of the system is lowered.

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Why Do Endothermic Processes Occur?

• Yet we know that in some processes, like the
  dissolution of NH4NO3 in water, heat is absorbed,
  not released.

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Enthalpy Is Only Part of the Picture

• The reason is that increasing the disorder or
  randomness (entropy) of a system tends to lower
  the energy of the system.

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Enthalpy Is Only Part of the Picture

• So even though enthalpy may increase, the
  overall energy of the system can still decrease
  if the system becomes more disordered.

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• Just because a substance disappears when it comes
  in contact with a solvent, it doesnt mean the
  substance dissolved.
• Dissolution is a physical changeyou can get back
  the original solute by evaporating the solvent.
• If you cant, the substance didnt dissolve, it
  reacted (note in some aqueous situations the
  solute becomes hydrated this involves a
  chemical change with addition of water of
  crystallisation).

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Types of Solutions

• Saturated
• Solvent holds as much solute as is possible at
  that temperature.
• Dissolved solute is in dynamic equilibrium with
  solid solute particles.

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Types of Solutions

• Unsaturated
• Less than the maximum amount of solute for that
  temperature is dissolved in the solvent.

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Types of Solutions

• Supersaturated
• Solvent holds more solute than is normally
  possible at that temperature.
• These solutions are unstable crystallization can
  usually be stimulated by adding a seed crystal
  or scratching the side of the flask.

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Factors Affecting Solubility

• Chemists use the axiom like dissolves like
• Polar substances tend to dissolve in polar
  solvents.
• Nonpolar substances tend to dissolve in nonpolar
  solvents.

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Factors Affecting Solubility

• The more similar the intermolecular attractions,
  the more likely one substance is to be soluble in
  another.

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Factors Affecting Solubility

• Glucose (which has hydrogen bonding) is very
  soluble in water, while cyclohexane (which only
  has dispersion forces) is not.

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Factors Affecting Solubility

• Vitamin A is soluble in nonpolar compounds (like
  fats).
• Vitamin C is soluble in water.

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Gases in Solution

• In general, the solubility of gases in water
  increases with increasing mass.
• Larger molecules have stronger dispersion forces.

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Gases in Solution

• The solubility of liquids and solids does not
  change appreciably with pressure.
• The solubility of a gas in a liquid is directly
  proportional to its pressure above the liquid.

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Henrys Law

• Sg kPg
• Where
• Sg is the solubility of the gas
• k is the Henrys law constant for that gas in
  that solvent
• Pg is the partial pressure of the gas above the
  liquid.

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Temperature

• Generally, the solubility of solid solutes in
  liquid solvents increases with increasing
  temperature.

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Temperature

• The opposite is true of gases
• Carbonated soft drinks are more bubbly if
  stored in the refrigerator.
• Warm lakes have less O2 dissolved in them than
  cool lakes.

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Expressing Solution Concentration

• Mass Percentage
• Parts per million (ppm)
• Parts per billion (ppb)

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• Mole fraction (X)
• Molarity (M mols/L)
• Because volume is temperature dependent, molarity
  can change with temperature.
• Molality (m mols/kg)
• Because both moles and mass do not change with
  temperature, molality (unlike molarity) is not
  temperature dependent.

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Changing Molarity to Molality

• If we know the density of the solution, we can
  calculate the molality from the molarity, and
  vice versa.

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Colligative Properties

• Changes in colligative properties depend only on
  the number of solute particles present, not on
  the identity of the solute particles.
• Among colligative properties are
• Vapor pressure lowering
• Boiling point elevation
• Melting point depression
• Osmotic pressure

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Vapor Pressure

• Because of solute-solvent intermolecular
  attraction, higher concentrations of nonvolatile
  solutes make it harder for solvent to escape to
  the vapor phase.
• Therefore, the vapor pressure of a solution is
  lower than that of the pure solvent.

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Raoults Law

• PA XAPA
• where
• XA is the mole fraction of compound A
• PA is the normal vapor pressure of A at that
  temperature
• NOTE This is one of those times when you want
  to make sure you have the vapor pressure of the
  solvent.

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Boiling Point Elevation and Freezing Point
Depression

• Nonvolatile solute-solvent interactions also
  cause solutions to have higher boiling points and
  lower freezing points than the pure solvent.

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Osmosis

• Some substances form semipermeable membranes,
  allowing some smaller particles to pass through,
  but blocking other larger particles.
• In biological systems, most semipermeable
  membranes allow water to pass through, but
  solutes are not free to do so.

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Osmosis

• In osmosis, there is net movement of solvent
  from the area of higher solvent concentration
  (lower solute concentration) to the area of lower
  solvent concentration (higher solute
  concentration).